The present invention relates to a switch arrangement and, more particularly, to a heat switch which includes a base body, connection elements, insulated from one another and fixed to the base body, a movable contact member, at least one fixed contact member, and a bimetal switch member whereby the contact members are connected so as to be electrically conductive with the respective connection elements and, at a predetermined change of temperature, the contact members may be connected or separated from each other by the bimetal switching member.
Heat switches of the aforementioned type are employed in a host of electrical instruments such as, for example, electric heating apparatus, electric motors, transformers, etc. to monitor the temperature of the instrument in question and especially to prevent damage which may be caused by unacceptable heating, i.e. ore heating.
In principle, switches of the aforementioned type function in such a manner that, at a specific change of temperature, i.e. a temperature rise above a predetermined temperature or a temperature drop below a predetermined temperature, the bimetal member, preferably constructed in a conventional manner as a bimetal snap plate, changes its configuration and thereby the movable contact member is actuated in such a way that a circuit that is to be monitored is opened or closed. In most applications the device is so constructed that in a low temperature setting the movable contact member is applied to the fixed contact and the circuit therefore is closed, and in a high temperature setting the circuit is interrupted so that there is no further delivery of energy to the apparatus or instrument being controlled. After the temperature returns below the predetermined temperature the circuit correspondingly is closed again. However, it is also possible to provide a switching arrangement in which, for example, a signal circuit is closed at the high temperature setting and, with a number of fixed contact members, both functions may be combined.
Heat switches of the aforementioned type have been constructed in many varied constructions. From practical experience, the simplest form of such a heat switch is one in which the fixed contact member is constituted by a simple rigid contact tongue and the movable contact member is formed by the bimetal member itself. While this construction is inexpensive, there is a disadvantage in that such a construction lacks precision in its response time since the bimetal member has the current that is to be switched passing through it, so that in any case only low currents can be switched.
To avoid the above-noted disadvantage, in German Pat. No. 21 21 802, a heat switch arrangement is proposed wherein a movable contact member is provided and is adapted to be actuated by a bimetal member through which no current or in any case a low current passes. In this arrangement, there is provided a spring snap plate with a contact piece forming a movable contact, said spring snap plate being actuated by a bimetal snap plate. The connection element and fixed contact member of this proposed heat switch arrangement are constituted by parts of the housing. In this arrangement, the spring snap plate must have a lower spring force than that of the bimetal snap plate in order to enable it to be actuatable thereby, and consequently, it has to have a relatively small cross section so that strong or large currents cannot be switched by this switch.
In German Pat. No. 17 90 103 another heat switch arrangement is proposed wherein a fixed contact member with each connecting element and a movable contact member is made as a bridge that is adapted to be actuated by a bimetal plate and held in its respective position by an annular spring. In this arrangement, the movable contact member can be made with a relatively large conducting cross section. However, a disadvantage of this arrangement resides in the fact that the manufacturing of this switch arrangement requires a relatively great outlay of material and labor. Additionally, the resultant heat switch is rather large.
Accordingly, an object of the invention is to solve the problem of producing a heat switch of the above-described type which is compact, but allows the switching of large currents.
A further object is to produce a heat switch that can be manufactured with a minimum outlay for labor and material.
The above objects are achieved according to a preferred embodiment of the invention by providing a heat switch with a movable contact member that is formed so as to be relatively stiff in bending and is connected by an electrically conducting articulated connection with a connection element, the articulated connection comprising a fixed part shaped on the connection element and a movable joint part shaped on the contact member, and the joint parts being rotationally slidable and applied so as to be electrically conductive and pressed against each other by spring force.
In contrast to known heat switches, in the heat switch according to the present invention the movable contact member is not actuated by way of its elastic deformability, but instead is achieved by an articulated connection to the connection element that allows a through-passage of electrical current. Moreover, the relatively stiff movable contact member can be made without difficulty with a cross section that is selected to carry the necessary electric current that is to be switched. The same is true with respect to the connection element and fixed contact member.
Also, it is advantageous, in accordance with the preferred embodiment, if the articulation parts themselves are shaped in the simplest way on the connection element and the movable contact member, respectively, and are pressed against each other by the force of a spring, so that both a kinematic holding together and low transmission resistance are ensured. Since the articulation connection in any case develops low friction forces, but no elastic recall force, actuation can be effected in the simplest way with the conventional bimetal member formed, advantageously, by a bimetal snap plate. Thereby, it is also possible to achieve uncomplicated, space-saving heat switch construction.
The articulation parts advantageously comprise cylindrical surface sections with concentric application surfaces that are respectively formed on the connection element and the movable contact member, respectively. Of these concentric application surfaces, one surface is convex and the other surface has the same curvature, but is concave. Preferably, the connection element and the contact member are stamped and bent as sheet parts so that the articulation parts can be made in a simple manner as cutouts from a cylindrical surface of suitable radius, and bent.
Actuation of the movable contact member by the bimetal member, in accordance with the present invention, is effected in that the bimetal member is connected along an edge by positive engagement of, for example, suitable projections, recesses, etc., with a free end of the movable contact member, while the base body presents a bearing for the edge region of the bimetal member located opposite the movable contact member.
In accordance to further features of the invention, to create unequivocal relationships for the flow of force, it is generally recommended that a bracing for the middle zone of the bimetal member be provided. In an embodiment that is particularly advantageous, in consideration of function and outlay for construction, the bracing is provided on the articulated connection, especially on the articulation part of the movable contact member, so that the bimetal member also exerts the spring force with which the articulation parts are pressed against each other.
An especially space-saving, readily mounted construction is produced by constructing the fixed contact member with a plane surface and so as to present a central recess. As a result, the fixed contact member can be stamped and bent in a simple manner together with the connection element as an integral sheet form, whereby the actual contact member with the recess forms a closed square. In such a construction, the contact element of the movable contact member extends, with an inner connection extension that is likewise shaped in one piece, essentially in a plane of the fixed contact member and inside its recess where, in consideration of insulation and dielectric strength, suitable distances are obviously to be maintained.
A still further advantageous feature is achieved by the recess of the fixed contact member being so made that, at the same time, a fastening element (such as a rivet or the like) also runs in an electrically insulated manner, through the recess with which the connection element of the movable contact member is connected with the base body. The fixed contact member and/or its connection element may be cast or impressed directly into the base body, and it also has been found to be beneficial if the base body is provided with positive-engaging shapes in the form of projections, depressions, etc., that are engaged by the fixed contact member and that engage the connection extension to the movable contact member so as to achieve an unequivocal fixing of the components of the switch that simplifies assembly, and at the same time counteracts leakage currents.
The fixed articulation part, in a preferred form, is made as a cylinder surface section that is bent on the connection extension, that extends convexly as a cylinder surface section from the plane of the fixed contact member and is braced on a projection of the base body. As noted above, the movable articulation part is thereby set on with a concave surface of application with equal curvature and pressed elastically by the bimetal member.
In accordance with another heat switch embodiment of the present invention preferably a set spring is provided that has less spring force than that of the bimetal member in snapping over. The set spring is connected on one side with the base body, and a free end thereof engages on the free end of the movable contact member. The set spring thereby, essentially, has the configuration of a leaf spring that engages the contact member by forms that are suitable for positive engagement (such as projections, bends, recesses, etc.). Also, preferably, the set spring is made by suitable stamping as a spring snap element. By this arrangement the advantage is achieved that the retaining force in at least one of the two switch positions is applied by the set spring, and the bimetal member only induces the switchover and otherwise is free of stress, whereby disadvantageous fatigue, that would affect the response behavior of the bimetal member, is avoided. The set spring can also constitute the bracing for the contact member that is movable with respect to the edge zone of the bimetal member. The positive force engagement of the bimetal member on the movable contact member as provided by the invention occurs directly in that the bimetal member engages on the free end of the set spring. Moreover, there is the advantageous possibility, in at least one of the two switch positions, to apply the spring force wherewith the articulation parts are pressed against each other by means of the set spring. This application of spring force may be accomplished by a biasing effect created, for example, by suitable bending.
It has also been found to be advantageous, in accordance with a preferred feature, if the bimetal member is disposed without clamping, i.e. as if it were "flying" and having only lateral guidance, the lateral guidance being effected by a switch housing and/or by projections of the base body. The set spring with the arms that are shaped on integrally or subsequently fastened on can thereby have a more or less cruciform outline. This lateral guidance feature is especially recommended when heat switches according to the invention are delivered open, for subsequent introduction into a housing.
It is noted that the heat switch according to the invention, among other things, offers the advantage that, with small dimensions, large currents can be switched. For this reason a heat switch according to the present invention is quite specifically suited for a new arrangement for overload protection of electric motors, wherein the housing of the heat switch is made as a winding cover for fixation of current windings of an electric motor in winding grooves of a stator and/or rotor as described in my copending U.S. Application Ser. No. 145,576 having the same effective filing date as this application and entitled "Electric Motor with Thermal-Protection Switch", and now abandoned.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.